1. Field of the Invention
This invention relates generally to liquid level gauging systems, and more particularly to those employing a light-conducting rod, light source, and light-receiver, and wherein the reflective properties at the surfaces of the rod change as it becomes immersed in the liquid being gauged, such change being detected by the light-receiver.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
A typical apparatus of the type noted above is illustrated and described in U.S. Pat. No. 3,448,616 dated June 10, 1969, and issued to W. Wostl et al. The patent discloses a light-conductive rod which employs the well-known physics "law of refraction", to measure liquid levels. The rod is tapered, and constituted of transparent material. There is provided a source of light and a photodetector at the upper end of the rod, which is intended to be mounted vertically in a tank or vessel. As the level of liquid in the tank rises, immersing a greater portion of the lower end of the rod, there is less light reflected from the rod-liquid interface, and consequently the intensity of the light received by the photodetector is reduced. The photodetector thus provides an indication related to that portion of the rod which is immersed at any particular time, this in turn providing an indication of the level of the liquid in the tank.
Another arrangement is illustrated in U.S. Pat. No. 3,299,770 dated Jan. 24, 1967, and issued to W. Witt et al. The disclosed device is employed to measure liquid levels in a conduit; it incorporates a transparent rod of substantially uniform diameter, having a light-reflecting surface at its lower end. Measurement of the liquid level is made by directing a beam of light downwardly through the rod, and detecting a portion of the beam that is reflected from the rod-liquid interface, by means of a photocell. The intensity of the reflected beam is a function of the liquid level in the conduit.
Similar devices are illustrated in U.S. Pat. Nos. 3,766,395; 3,834,235; 4,134,022; 4,242,590; 4,354,180; and 4,458,567.
Also, U S. Pat. Nos. 3,917,411; 3,932,038 and 3,977,790 illustrate various types of transparent rods that utilize the law of refraction for monitoring liquids. In contrast to the devices which are the subject of the patents listed in the previous paragraph, these three patents show gauges that measure the density or specific gravity of a liquid. In such cases, the photosensitive devices associated with the rods receive an amount of light which varies with such density or specific gravity.
In most of the above patented devices, the transparent rod was exposed for a major portion of its length and was thus susceptible to bumping or jarring, especially during handling or installation. Since the materials employed were usually plastic or glass, there existed a danger of fracture, breakage, or chipping of the transparent portion, which would render the device inoperative.
In addition, most of the liquid level sensors shown in the above identified patents were intended for installation in a substantially vertical position on a vessel housing; that is, they were usually mounted on an upper cover thereof with the transparent part extending vertically downward. While this was satisfactory for many installations, space requirements did not always permit mounting in this manner. In particular, devices intended to monitor oil level in a motor vehicle's oil pan could not be mounted directly over the pan, since this area is occupied by the engine's pistons and cylinders.
Accordingly there has existed a need for a gauge that would be both rugged and reliable, and capable of being mounted on a vertical wall of an oil pan. Absence of moving parts of any kind was also felt to be an important consideration, due to the fact that mechanical floats and switches associated therewith were in some cases considered to be unreliable. In addition, mechanical gauges have not found acceptance in oil monitoring systems for motor vehicles, mostly due to the wide range of temperatures occurring at the oil pan, together with the harsh environment involving mechanical shock, and vibration of the engine during operation.
One type of gauge especially intended for monitoring the level of oil in an oil pan is shown in U.S. Pat. No. 3,939,470. This patented device utilized two sensors comprising thermal variable resistance elements, the first element being located in one vertical wall of the pan, with the other being on an adjacent wall, perpendicular to the first. This arrangement constituted an attempt to compensate for errors which would occur where oil level readings were taken at times that the vehicle was not disposed on a level surface. For example, when the vehicle was parked on a hill and where a single sensing element was employed, a false reading would likely be indicated, depending on the location of the sensing element about the pan walls, and the direction of tilting of the vehicle.
While this patent appears to address the problem involving erroneous readings resulting from tilting of the vehicle, the solution of providing dual sensing elements constitutes a compromise at best. In addition, as presently understood, it is believed that false readings could still occur if the vehicle were either tilted toward one side or tilted from front to back, and vice-versa.
Finally, it is believed that optical sensors of the type incorporating a conical tip which becomes immersed in the liquid being gauged have, up to the present time, been designed to operate with the axis of the cone lying along a vertical line. We have discovered that a surprising and unexpectedly advantageous result can be obtained if a sensor employing a conical tip is mounted with the axis of the cone substantially horizontal, for example, as where the sensor is mounted in a vertical wall of the oil pan of an automotive vehicle. Further, an unusually high degree of resolution can be obtained, whereby excursions into an area of the sensor unit on the order of 3 millimeters in length can be detected, with excellent repeatability and reliability.